1. Field of the Invention
The present invention relates to a reflection type variable optical attenuator.
2. Related Art
Optical communication systems or optical measuring systems need a variable optical attenuator for controlling the amount of transmitted light. A typical example of such a variable optical attenuator is an opposed type that has a polarizer and an analyzer placed on the optical axis in front of and behind (on the input and output sides) variable polarization rotation means (a Faraday rotation angle varying device). The variable polarization rotation means has an electromagnet, etc., applying a magnetic field externally to a Faraday element (such as a magnetic garnet single crystal film having a Faraday effect) and controls the Faraday rotation angle of light passing through the Faraday element by varying the applied magnetic field. By this control of the Faraday rotation angle, the variable optical attenuator variably controls the attenuation of light.
In these years, as wavelength multiplex communications are coming into practical use, an optical attenuator is provided for each wavelength to make insertion loss even for the wavelengths. In such a usage, it is particularly important that the optical attenuator is inexpensive to produce and can be made smaller in size. Conventional variable optical attenuators of the opposed type need a broad space for fibers to be laid in, except where their input and output are arranged opposite each other, resulting in less degrees of freedom in mounting.
As technology to solve this problem, there has been proposed a reflection type variable optical attenuator wherein a separation/combination birefringent element, a lens, and a mirror are arranged in that order and an input port and an output port are provided on the front side of the separation/combination birefringent element and wherein the attenuation of incident light is adjustable by controlling the rotation angles of the polarization directions by use of variable polarization rotation means provided in between the separation/combination birefringent element and the mirror. Refer to Japanese Patent Application Laid-Open Publication No. 2003-107420. This reference also discloses technology wherein a combination of a base film Faraday element whose Faraday rotation angle varies according to a variable magnetic field and a compensation film Faraday element whose Faraday rotation angle does not vary depending on the variable magnetic field is provided thereby achieving a high attenuation and widening the range in which the attenuation is variable.
In such a variable optical attenuator, the Faraday rotational directions of the base film Faraday element and of the compensation film Faraday element must be opposite to each other. Thus, the composition is different between the base film Faraday element and the compensation film Faraday element, and thus producing conditions are different. Hence, the base film Faraday element and the compensation film Faraday element are separately produced. Therefore, the base film Faraday element and the compensation film Faraday element are different in wavelength-dependent loss and temperature-dependent loss, and a difference in Faraday rotation angle occurs depending on conditions of wavelength and temperature. Thus, the maximum attenuation peak shifts against the drive current, resulting in a bad effect on optical characteristics of the variable optical attenuator such as the reduction of the attenuation range. Therefore, it is difficult to widen the range of operating temperatures and the range of operating wavelengths.